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This is an old revision of this page, as edited by 64.12.116.65 (talk) at 12:17, 12 April 2004. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Can we fix the h-bar to use the Unicode as seen on the Plancks' Constant page? The display as it is at the moment appears to me as an h with a line through the curved n-like part of the stroke, whereas it should the bar should cross the h in the upper part above the 'n' - EddEdmondson 22:42 Feb 4, 2003 (UTC)

"So the complete set is based on five (not three) fundamental physical constants: G, c, h-bar, k, and e"

Two issues. First, h-bar is h/2π. h is the more fundamental of the two formulations, because the true value of π is dependent upon the geometry of the universe (i.e. if the universe is non-Euclidian, then you will need to change the value of π used in all physics formulations accordingly so that it still fits the definition 2π = C/r).

This is not correct: the value of Pi has nothing to do with the geometry of spacetime. It is a mathematical constant, defined by some convergent series. In Euclidean space it happens to be the ratio of circumference to diameter, and in non-Euclidean spaces (like our universe), that's not universally true. But the physical equations always use the precise mathematically defined value, not some experimentally determined number. AxelBoldt

Second, e (the charge of the electron, not the base of the natural log, right?), makes for a bad fundamental unit since there are quarks with smaller units of charge. This objection only holds, of course, if the Planck units are intended to represent the indivisible quanta of each measurement type.

Thanks for the nice questions. You say:

<<objection only holds, of course, if the Planck units are intended to represent the indivisible quanta of each measurement type.>>

That is right! What you say is correct. Therefore the objection does not hold. Because when Planck defined them in 1899 (and Stoney did part before him in the 1870s) the units were only intended to be universal natural units (making the most widely used universal constants unity) and were NOT intended as "indivisible quanta". e is the natural unit charge Stoney discovered in the 1870s before anybody knew there was a particle. He called the charge unit "electron" then when his friend J.J.Thomson discovered there was actually a particle with the unit charge on it (1897) he used Stoney's name to name the particle. That amount of charge is our fundamental constant for charge. It is great that there are quark charges of 1/3 and 2/3 e!!! Murray Gel-man who invented quarks did not ask to change e to 1/3 e. It's fine. e stays e, the charge on the electron. and we can have fractions of it.

When Planck defined Planck units in 1899 in effect he used h-bar. He got the units you get using h-bar. The values Planck gave for the basic units in that 1899 paper are amazingly close to the ones we use today. Exactly how and why I can't explain even though I have read the relevant parts of his 1899 paper! Somehow h-bar is at the historical root and not h. Some people think h-bar is "more fundamental". Maybe Planck thought that then. It is perhaps useless to discuss which is more fundamental! The thing to remember is that if you say PLANCK units those are the historical ones which he defined and which have gradually come into use over the past century. We cannot change them. You or I can only make up our OWN units and try to get physicists to be interested in them.

Anyway. Planck units use h-bar, for whatever reason. Also the value of h-bar does not depend on space-time geometry because h-bar is physically meaningful and can be measured. You measure h-bar. You don't need to go around measuring h and dividing by 2 pi. Another thing: locally "pi is pi". It is a mathematically defined number that works locally. Indeed there is a lot of evidence that spacetime has negative curvature so that for VERY LARGE circles C/r could be bigger than 2 pi. But at your scale and mine and at the scale of atoms pi is not worried by this. Non-Eucl. geometry has an idea of local flatness which is compatible with large-scale curvature and our old friend pi works in local flat neighborhoods.

Hope this helps.

Removed this from the main article.

Planck units are not 'alternative physics' and, in particular, the Planck force (about 10 tons) which is simply the unit force going along with the other Planck units is not, 'alternative physics'. On the contrary, given the units' growing importance in string theory and cosmology, they are mainstream.

Removed this from the main article.

The SI units are increasingly defined in terms of fundamental constants also, but unlike the Planck units their definition includes arbitrary numbers which are not powers of ten, which are present only for historical reasons.

From a physics stand point, powers of ten are just as arbitrary and historical as anything else.

Someone seems to have been getting a bit defensive about some edits made in gravitational constant. I've fixed that article, which makes this defense a bit redundant.

"Quantum physics states that it is impossible to divide a unit of measurement (length, mass, time, temperature) into segments smaller than the Planck constant, while obeying the known laws of physics."

Given that the Planck temperature is 1.4x10K, this seems just a bit contradictory. Maybe someone who knows this stuff better can explain how this applies to temperature? -- JohnOwens 08:34 Mar 24, 2003 (UTC)

Now that I think about it, ditto for the mass. -- JohnOwens 08:46 Mar 24, 2003 (UTC)

One important property of Planck units is that at Planck temperature the kinetic energy of "typical" particles (or heavier than the Planck mass?) is such that their de Broglie wavelength is smaller that their Schwarzschild radius (= critical radius of black holes). This was when the Universe was younger than 1 Planck time (see Timeline of the Big Bang).

Since when has ${\displaystyle \hbar }$ been called Dirac's constant? Phys

Since this article was written?

A limit wavelength photon can be defined from the Planck length unit. This wavelength photon has the energy density to produce a pair of black holes such that each black hole would have a photon capture radius (3Gm/c squared) equal to the photon wavelength divided by two pi. This limit wavelength is defined as (3/2) exponent 1/2 times (2 pi) times ( Planck length). The limit wavelength is (3pi hG/c cubed) exponent 1/2. The square root of the product of this wavelength and the length (2 pi) squared times (c times one second) meters is 2 pi (3pi hG/c) exponent 1/4. This is a photon wavelength that has energy equal to the mass energy of one electron plus one positron.The electron Compton wavelength is 4 pi (3pi hG/c) exponent 1/4. The electron mass will then be (h/4pi c) times (c/3pi hG)exponent 1/4. The value of the gravitational constant is 6.6717456 x 10 exponent -11 if no small corrections apply. See Talk:Time dilation. Don J. Stevens 4/10/04